Agriculture Reference
In-Depth Information
Table 18.6
Environmental Conditions Determined for Curing of Yam Tubers to Prolong Shelf Life.
Species
Temperature (°C)
Humidity (%)
Duration (days)
Reference
D.alata
32
90
4
Gonzalez and Rivera (1972)
D.rotundata
25-30
55-82
5
Adesuyi (1973)
D. esculenta
26-28
high
5-7
Martin (1974)
D. dulbifera
D. rotundata
25-40
95-100
—
Been
et al
. (1976)
D. caynensis
D.rotundata
26
92
11-15
Nnodu (1986)
D. caynensis
36-40
91-98
—
Thompson
et al
. (1977)
The process of wound-healing in yam
As for sweet potato and cassava roots, yam tubers are able
to wound-heal. Passam
et al
. (1976) examined the physio-
logical processes involved and concluded that a layer of
suberin formed under the wound, below which a wound
periderm was formed. This process is accompanied by an
increased metabolic rate. Several workers have reported
the advantages of storing cured tubers over uncured tubers.
Table 18.6 summarises the findings of a number of studies
to determine the optimum conditions for curing for a range
of species. There seems to be agreement on the use of high
humidity (>70°C) but there is still a wide range in suggested
optimum curing temperature (25-40°C) and curing
duration (2-15 days).
Much is still to be discovered regarding the mechanisms
that control wound healing in yam, in particular how the
process may be influenced by factors such as tuber
maturity at harvest and duration of storage of tubers. The
optimum age at harvest for tubers to be stored for a long
period is an important factor for yam production. The time
of maturity varies considerably between cultivars (Degras
1993) but little is known about the effect of maturity at
harvest on curing and therefore on the storability of yams.
It is also important to investigate if yams stored over a
period of time but later sustain some damage due to
transportation and handling can be cured before export.
The physical effects of curing on tuber skin strength,
toughness and fracture (skin strength, skin elasticity and
tissue integrity) is also worth studying as this impacts on
the level of damage on cured tubers as they are transported
or inspected in storage.
Table 18.7
Dormancy Periods of the Major Edible
Yams.
Length of dormancy
(weeks)
Yam species
Locality
D. alata
Caribbean 14-16
West Africa 14-18
D. rotundata
West Africa 12-14
D. cayenensis
West Africa 4-8
D. esculenta
West Africa 12-18
Caribbean 4-8
D. trifida
Caribbean 4
Source: Information from Passam (1982).
has been observed that dormancy break is associated with
an increased susceptibility to rotting. The rate of respiration
is also very dependent on the state of dormancy. It has been
observed that the rate of respiration at harvest (15 and
29 ml CO
2
/Kg fresh wt/h at 25 and 35°C respectively)
drops to 20-30% with the onset of dormancy, but returns to
harvest levels at dormancy break. During ambient storage
these rates of respiration would lead to significant loss.
Thus extension of dormancy is important to improve
storability. On the other hand, for breeding programmes it
is important to find ways to break dormancy in order to
speed up improvement programmes.
The dormancy period can be defined as a period of
reduced endogenous metabolic activity during which the
tuber shows no intrinsic or bud growth, although it retains
the potential for future growth. Length of dormancy varies
considerably by species and variety. It is also affected by
temperature moisture, oxygen and CO
2
content of the
storage atmosphere (Diop & Calverley 1998). Table 18.7
shows the dormancy period for the main species of yam.
The length of dormancy almost certainly depends on the
length of the dry season in the region of evolution. Thus
The control of sprouting of yam tubers
Storage of yams tends to be limited by natural dormancy
(ranges from 4-18 weeks depending on variety/species).
Not only does sprouting directly affect yam quality, but it
